JPH08271209A - Light waveguide path type displacement sensor and hemispherical lens used for the sensor - Google Patents

Abstract

PURPOSE: To measure even the surface shape of a pipe inner surface and a hole side surface by providing a hemispherical lens for reflecting and focusing measurement light in the direction vertical to the longitudinal direction of a light waveguide path substrate. CONSTITUTION: A light waveguide path substrate A is provided with end faces 1a and 1b in parallel with a longitudinal direction and inclined end faces 1c and 1d, and consists of an electrooptical crystal substrate 1 and light waveguide paths 2 and 3. The waveguide path 3 has an edge part vertical to the end face 1d, and the waveguide path 2 forms a certain angle to the end face 1d. The substrate A is housed in a cylindrical sensor case 10 whose outer diameter is approximately 6mm, with the emission edge of the waveguide path 2 at the center of the case 10. A hemispherical lens 12 is provided at a position where the distance between the emission edge and the center of the spherical surface of the lens 12 is approximately 7.5mm on the emission light axis at approximately 6.05 degrees from the emission edge. By adjusting the angle between the normal of the reflection surface of the lens 12 and incident measurement light to be approximately 48 degrees, the measurement light is totally reflected on the reflection surface of the lens and is focused at a position which is approximately 3.2mm in a direction vertical to the longitudinal direction of the substrate A, thus the side surface of a hole with the inner diameter of at least about 7mm, can be measured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide type displacement sensor for measuring displacement of an object to be measured by optical interference and a lens used therefor.

[0002]

2. Description of the Related Art A Michelson interferometer constructed on an electro-optic crystal substrate such as lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ) does not require complicated optical system alignment and is small displacement. There is an optical waveguide type displacement sensor as a meter. In the Michelson interferometer, if there is a boundary surface perpendicular to the optical axis of the measurement light, the reflected light from the boundary surface is mixed with the reflected light from the object to be measured, and the measurement accuracy is deteriorated. Therefore, in the optical waveguide type displacement sensor, the input / output end face for measurement light is inclined to suppress the reflected light from the boundary face. Therefore, the measurement light emitted from the optical waveguide is not always parallel to the longitudinal direction of the optical waveguide substrate due to refraction. Since the incident direction of does not coincide with the longitudinal direction of the optical waveguide substrate, there is a problem that the assembly becomes difficult and the positioning of the displacement sensor with respect to the measured object becomes difficult. In order to solve this problem, there is a method of obliquely polishing one end surface of the gradient index lens. Examples of such a technique include, for example, Japanese Patent Application No. Hei 6-16544.
There is one proposed in.

[0003]

However, the above-mentioned optical waveguide type displacement sensor cannot be used for measuring the surface roughness and shape of the inside of a pipe, the side surface of a hole or recess of a cylinder or the like. Even if the optical waveguide type displacement sensor can be inserted into a pipe or a hole, the measuring light is condensed in a direction parallel to the longitudinal direction of the optical waveguide substrate. This is because the side surface of the hole cannot be irradiated.

The present invention has been made in view of the above problems, and an object thereof is to provide an optical waveguide type displacement sensor capable of measuring the surface roughness and shape of the inside of a pipe and the side surface of a hole. .

[0005]

In order to achieve the above object, an optical waveguide type displacement sensor of the present invention is such that at least one of the end faces of the optical waveguide substrate in the longitudinal direction is an inclined surface with respect to the longitudinal direction. An optical waveguide substrate having a measuring light input / output end formed on the slope, an optical fiber for guiding the light source light output from the light source to the optical waveguide substrate, and a light receiving device for receiving the interference signal light output from the optical waveguide substrate Optical waveguide type displacement provided with an optical fiber for guiding to an optical waveguide and a transmission line for applying a modulation electric signal to a modulation electrode for modulating the reference light and / or the measurement light provided on the surface of the optical waveguide substrate. The sensor includes a hemispherical lens for returning and condensing the measurement light in a direction perpendicular to the longitudinal direction of the optical waveguide substrate.

As another structure, between the measuring light input / output end surface of the optical waveguide substrate and the hemispherical lens, the measuring light obliquely emitted from the measuring light input / output end surface is completely parallel light. A gradient index lens that converts light into a non-collimated ray may be provided.

At least an optical waveguide and a directional coupler are formed on the surface of the optical waveguide substrate, and a light source light input end and an interference signal light output end are formed at one longitudinal end thereof. The measuring light input / output end and a reference light reflecting mirror are formed on the other end surface. The outer peripheral surface of the hemispherical lens is composed of a spherical surface and a plane including the center of curvature of the spherical surface.

[0008]

The light emitted from the light source is guided to the optical waveguide substrate through the optical fiber and is branched into two optical waveguides by the directional coupler in the optical waveguide substrate. It is guided to the measuring light input / output end surface which is formed such that the normal line of the input / output end surface for measurement has an angle θ _t . One of the lights enters the end face perpendicularly, is reflected by a reflecting mirror provided on the end face, and returns to the directional coupler again as reference light.

The other light propagates through the optical waveguide formed parallel to the longitudinal direction of the optical waveguide substrate and faces the end face. Therefore, the light enters the end face at an angle θ _t and is emitted as measurement light in the direction of the angle θ _r (see FIG. 3) shown in Expression (1).

Θ _r = sin ⁻¹ [n _w / n ₀ · sin (θ _t )] − θ _t (1) Here, the angle θ _r is the input / output of the measurement light with respect to the longitudinal direction of the optical waveguide substrate. Is the exit angle from the end face, n _w ,
n ₀ is the refractive index of the optical waveguide substrate and air.

The measurement light emitted from the measurement light input / output end face of the optical waveguide substrate then enters the spherical surface of the hemispherical lens.
If the hemispherical lens is arranged so that the measuring light passes through the center of the spherical surface of the hemispherical lens, the measuring light will enter perpendicularly to the incident surface, and thus refraction will not occur. Further, since the hemispherical lens is formed as a flat surface such that the lens bottom surface of the hemispherical lens includes the center of curvature of the spherical surface, by applying a reflective film to the lens bottom surface, the lens bottom surface becomes a reflection surface, and the measurement light is It will be reflected at the center of the sphere. Therefore, the measurement light is emitted perpendicularly to the emission surface of the hemispherical lens regardless of the reflection angle.

Representative examples of the direction of reflection of the measuring light by the hemispherical lens are shown in FIGS. 3 and 4. As shown in FIG. 3, the measurement light emitted from the measurement light input / output end face of the optical waveguide substrate is
On the lens reflecting surface that is the bottom surface of the hemispherical lens, in the same direction as the direction in which the measuring light is refracted at the end face of the optical waveguide substrate, when further bending the traveling direction, By setting the angle θ _n formed by the normal line of the lens reflection surface to an angle that satisfies the expression (2), the measurement light is reflected by the lens reflection surface in the direction perpendicular to the longitudinal direction of the optical waveguide substrate, and the DUT is measured. It is possible to emit and collect light toward.

Θ _n = (90 + θ _r ) / 2 (2) Further, as shown in FIG.
When the measurement light is bent in the direction opposite to the direction in which the measurement light is refracted at the end face of the optical waveguide substrate, the angle _θ n is made to satisfy the expression (3) so that the measurement light is emitted in the longitudinal direction of the optical waveguide substrate. And can be reflected in the vertical direction.

[0014] _{θ n = (90- θ r)} / 2 (3) Here, if _the refractive index of the hemispherical lens is denoted by nL, _angles θn
Is satisfied with the equation (4), the measurement light can be totally reflected by the lens reflecting surface.

[0015] nL · sin ₍ θn)> 1 (4) If total reflection can be performed, the reflection film applied to the lens reflection surface can be omitted, so the measurement light is reflected in the direction shown in FIG. 3 rather than the direction shown in FIG. who is that a large _angles .theta.n, easily totally reflects the cost of the semi-spherical lens can be made inexpensive.

The light reflected by the object to be measured enters the optical waveguide substrate following the path opposite to the above. The measurement light returning to the optical waveguide substrate interferes with the reference light, and the interference light is emitted from the optical waveguide substrate toward the light receiving element. The displacement of the object to be measured can be measured by detecting the change in the intensity of the interference light.

In order to return the measurement light in the direction perpendicular to the longitudinal direction of the optical waveguide substrate without using the present invention and to collect the light, a method of providing a condenser lens after the light is folded back as shown in FIG. As shown in FIG. 6, two methods are conceivable: a method of folding back after passing through a condenser lens. However, the former is
Since the measurement light emitted from the optical waveguide substrate gradually spreads and the beam diameter increases, it is necessary to use a condenser lens with a large aperture diameter to collect all the measurement light with a large beam diameter. Since a condensing lens having a large aperture diameter has a long focal length, there is a problem that the diameter of a measurable hole becomes large.

On the other hand, in the latter case, it is necessary to increase the distance from the condenser lens to the focal point. For this reason, the measurement light has to be condensed at a gentle angle, and an optical system in which only a part of the reflected light from the surface of the object to be measured returns to the optical waveguide, that is, an optical system with a small aperture must be adopted. Absent. As a result, S
This leads to deterioration of the N ratio and reduction of the measurable tilt angle of the measured object.

Further, both of the methods shown in FIGS. 5 and 6 require at least two optical components such as a reflecting mirror for reflection and a condenser lens. In these methods, two components are installed at the same time as the cost of the components. Since it is necessary, we will increase the cost of the assembly process.

[0020]

【Example】

(Embodiment 1) A first embodiment of the present invention will be described with reference to FIG. The optical waveguide substrate A used is substantially the same as that proposed in Japanese Patent Application No. 5-284843 filed by the applicant of the present application, such as lithium niobate (L _i NbO3) and lithium tantalate (L _i TaO3). Electro-optic crystal substrate 1
The optical waveguides 2 and 3 are formed on the. The optical waveguide substrate A has end faces 1a and 1b parallel to the longitudinal direction and two end faces 1c and 1d that are polished obliquely to the longitudinal direction.

The end portion of the optical waveguide 3 faces the inclined surface 1d at an angle of 0 degree, that is, perpendicularly, and the end surface 1d is provided with a metal or dielectric reflecting mirror 5 capable of efficiently reflecting the light of the optical waveguide 3. Has been formed. A modulation electrode 6 is formed on the optical waveguide 3, and a transmission line 7 for applying a modulation electric signal is connected to the modulation electrode 6.

On the other hand, the optical waveguide 2 is formed on the slope 1d at an angle. Reflected return light intensity from the light waveguide end face 1d is strongly dependent on the angle _of θt of the optical waveguide 2 and the end face 1d, _in the case of using a lithium niobate substrate θ
If t is 5 degrees or more, reflected return light can be sufficiently suppressed.
Refractive index of the lithium niobate substrate is about _2.2, [theta] t
_And it is referred to as 5 degrees θr will be 6.05 degrees.

The hemispherical lens 12 has a refractive _index nL = 1.
5, it has a shape obtained by dividing a spherical lens having a radius R = 1.5 mm in half, and the lens reflecting surface 12d is formed so as to include the center 12e of the spherical surface. Further, the lens entrance / exit surface 12
An antireflection film is applied to the spherical surface which is c. Here the refractive _index nL of the hemispherical lens is 1.5, them and Equation (4) from the measuring beam angle _degree θn of entering the normal lens reflecting surface 12d of the lens reflection surface 12d 42 degrees For example, the measurement light can be totally reflected by the lens reflection surface 12d.

The optical waveguide substrate A is housed in a cylindrical sensor case 10 having an outer diameter of 3 mm so that the emission end of the optical waveguide 2 is located at the center of the cylindrical sensor case 10. Hemispherical lens 12, exit end or _these optical waveguide 2
The center 12e of the spherical surface of the hemispherical lens 12 on the optical axis of the measurement light emitted in the direction of θr of 6.05 degrees.
_Distance L1 up has been installed at a position to be 7.5mm. Therefore, the distance d from the center axis of the cylindrical sensor case 10 in the radial direction to the center 12e of the spherical surface of the hemispherical lens is 0.79 mm. Here, the hemispherical lens 12 is fixed to the halved cylindrical member 11, and the cylindrical sensor case 1
The angle was adjusted by rotating the cylindrical member 11 in the round hole provided at 0. In this way, the semi-spherical lens 12, implementing _angles θn to 48 degrees, the measuring light is totally reflected by the lens reflection surface 12d, and the longitudinal and vertical direction of the optical waveguide substrate _A, L2 is 3.2mm Is focused at the position. Therefore, since the outer diameter of the cylindrical sensor case 10 is 3 mm, the working distance WD, which is the distance from the outer circumference of the cylindrical sensor case 10 to the focus of the measurement light, can be set to about 1 mm. This indicates that it is possible to measure the side surface of a hole such as a cylinder having an inner diameter of 7 mm or more.

The optical waveguide displacement sensor thus manufactured was used for phase modulation, and the step difference of the deposited film of the standard sample prepared by depositing Si on the Si substrate was measured. It was confirmed that it had the characteristics. Moreover, when the side wall of a circular hole with a diameter of 7 mm manufactured by electric discharge machining, which was difficult to measure in the past, was measured, it was 0.5 μm.
It was confirmed that the surface was processed with the surface roughness of.

Second Embodiment A second embodiment of the present invention will be described with reference to FIG. In the SELFOC lens 13, which is a kind of gradient index lens, the refractive index of the optical axis is 1.557, and the radial refractive index distribution n (r) is r, where the distance from the optical axis is r. n (r) = 1.557-0.
046 represented by ⁰ r2, the length of the optical axis direction is used as the 4 mm. Here, the optical waveguide substrate A having a normal line to the end face 13c
The _angles θS in the longitudinal direction so as to satisfy the equation (5) shown below, and polished to 10.6 degrees, the other end face 13d
Is 0 degree, and the end faces 13c and 13d are coated with a non-reflective coating.

[0027] n0.sin (6.05 ₊ .theta.S) ₌ nS.sin ₍ .theta.S) (5) _where n0 _and nS are 1.0 and 1.557, respectively, which are the refractive indexes of the optical axis of air and the SELFOC lens, respectively.

The SELFOC lens 13 has an end face 13
c is arranged so as to face the measurement light entrance / exit surface 1d of the optical waveguide substrate A. At this time, the measurement light emitted from the SELFOC lens 13 becomes parallel to the longitudinal direction of the optical waveguide substrate A, and the divergence angle of the measurement light is smaller than the divergence angle of the measurement light emitted from the optical waveguide substrate A. And becomes a quasi-collimate ray that is close to perfect parallel light. For this quasi-collimate ray, the same hemispherical lens 1 as described in Example 1
2 was placed such _that L1 was 6 mm. Further, the hemispherical lens 12 was mounted so that the angle formed by the normal line of the lens reflecting surface 12d of the hemispherical lens 12 and the longitudinal direction of the optical waveguide substrate A was 45 degrees. In this case, since the _angles θn of 45 degrees, in the direction of the longitudinal direction and the vertical of the optical waveguide substrate _A, L
The measurement light is condensed at a position where 2 is 3.6 mm. Therefore, when the outer diameter of the cylindrical sensor case 10 is 3 mm, the working distance WD is 0.6 mm, and it is possible to measure the side wall of the hole having the inner diameter of 6.6 mm.

The length of the SELFOC lens in the optical axis direction is 6.46 mm (0.25 pitch) so that the measurement light emitted from the SELFOC lens 13 is completely collimated.
Then, the condensing characteristic is improved and the lateral resolution is improved. In this case, however, in this case, the end surface 1 of the SELFOC lens 13 is
Since the reflected return light from 3d is condensed on the measurement light input / output end face 1d of the optical waveguide substrate A and returns to the optical waveguide 2, there arises a problem that the measurement accuracy is deteriorated. As described above, the length of the SELFOC lens 13 in the optical axis direction is 4 m.
When m, the reflected return light has a beam diameter of 250 μm at the measurement light entrance / exit end face 1d of the optical waveguide substrate A, so that the amount of light returning to the optical waveguide 2 is small and the measurement accuracy is not deteriorated.

[0030]

As described above, by using the present invention, it is possible to realize an optical waveguide type displacement sensor capable of measuring the surface roughness and shape of the side surface of a hole having a diameter of 6.6 mm, for example. At the same time, since the measurement light can be condensed and the optical path can be bent by one component, the number of optical components and the assembly cost can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a second embodiment of the present invention.

FIG. 3 is a diagram showing an example of an arrangement direction of hemispherical lenses and a reflection direction of measurement light.

FIG. 4 is a diagram showing another example of an arrangement direction of hemispherical lenses and a reflection direction of measurement light.

FIG. 5 is a schematic view showing an example of an optical waveguide type displacement sensor using a measurement light folding prism and a condenser lens.

FIG. 6 is a schematic view showing another example of an optical waveguide displacement sensor using a measuring light folding prism and a condenser lens.

[Explanation of symbols]

 A Optical waveguide substrate L Measuring light  A hemispherical lens from the input / output end face for measurement light of the L1 optical waveguide substrate.
Distance to the center of the spherical surface  Distance from the center of the spherical surface of the L2 hemispherical lens to the focal point d of the hemispherical lens from the central axis of the cylindrical sensor case
Distance to center of spherical surface WD Working distance F1, F2 Optical fiber 1 Lithium niobate crystal substrate 1a, 1b, 1c, 1d Optical waveguide substrate end face 2, 3 Optical waveguide 4 Directional coupler 5 Reflector 6 Modulation electrode 7 Transmission Line 10 Cylindrical sensor case 11 Cylindrical member 12 Hemispherical lens 12c Lens entrance / exit surface (spherical surface) 12d Lens reflection surface 12e Center of spherical surface 13 SELFOC lens 13c, 13d SELFOC lens end surface 14 Folding prism 15 Focusing lens

Claims (4)

[Claims] 1. An optical waveguide substrate in which at least one of the two end faces in the longitudinal direction is inclined with respect to the longitudinal direction and a measuring light input / output end is formed on the inclined face, and light source light output from a light source is provided. An optical fiber that guides the optical waveguide substrate, an optical fiber that guides the interference signal light output from the optical waveguide substrate to a light receiving device, and a reference light and / or a measuring light provided on the surface of the optical waveguide substrate In the optical waveguide type displacement sensor including a transmission line for applying a modulation electric signal to the modulation electrode, a hemisphere for returning and condensing the measurement light in a direction perpendicular to the longitudinal direction of the optical waveguide substrate. An optical waveguide type displacement sensor, which is equipped with a lens. 2. The measuring light obliquely emitted from the measuring light input / output end surface between the measuring light input / output end surface of the optical waveguide substrate and the hemispherical lens is converted into a quasi-collimate light beam which is not a perfect parallel light. The optical waveguide type displacement sensor according to claim 1, further comprising a gradient index lens for changing the refractive index distribution type lens. 3. An optical waveguide and a directional coupler are formed on the surface of the optical waveguide substrate, and a light source light input end and an interference signal light output end are provided at one longitudinal end of the optical waveguide substrate. The optical waveguide type displacement sensor according to claim 1 or 2, wherein the measurement light input / output end and a reference light reflecting mirror are formed on the other end surface. 4. The hemispherical lens according to claim 1, wherein an outer peripheral surface is formed of a spherical surface and a plane including a center of curvature of the spherical surface, and the refractive index is larger than 1.